The relative importance of buffering and brine inputs in controlling the abundance of Na and Ca in sedimentary formation waters

Marine and Petroleum Geology 28 (2011) 1242e1251

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The relative importance of buffering and brine inputs in controlling the abundance of Na and Ca in sedimentary formation waters Stephanie Houston a, b, Craig Smalley b, Adam Laycock a, Bruce W.D. Yardley a, * a b

School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK BP Exploration, BP International Centre for Business and Technology, Sunbury-upon-Thames, Middlesex, TW16 7LN, UK

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 July 2010 Received in revised form 24 February 2011 Accepted 4 March 2011 Available online 16 March 2011

The concentration of Ca in the formation waters of petroleum reservoirs can play a major role in influencing the outcome of a number of processes that are of great significance to the oil industry. For example, formation water Ca concentration affects the risk of carbonate scale formation during production. In order to better understand the concentrations of Ca in formation waters, we have investigated the chemistries of formation waters from a range of onshore and offshore basins worldwide, using published sources, as well as unpublished data held by BP. Although calcium and sodium are the principal cations in almost all formation waters they vary enormously in their relative proportions. We have identified three distinct trends on a plot of XCa (Ca/(Na þ Ca)) against Cl. Most data lie on a high-Ca trend, here termed Trend 1, and show an increase in XCa with salinity. We interpret this as tracking equilibration with Ca and Na-bearing minerals, with the ratio (mol Ca/mol Na2) remaining approximately constant irrespective of salinity for chloride-dominated fluids. At very high salinities, Br-enriched bittern brines that have taken part in dolomitisation lie at the Cl-rich end of this trend. Some brines remain Nadominated up to very high salinities and define a distinct low-Ca trend, Trend 2. These are associated with dissolution of halite beds and are interpreted to arise when the amount of Na in the pore fluid greatly exceeds the amount of Ca available in minerals. We refer to such brines as mass-limited; the sparsity of Ca in the rock-fluid system constrains XCa to a low value. Remarkably few brines lie between these trends. Finally, dilute formation waters show very large variations in XCa and may have bicarbonate as the dominant anion. They define a distinct low-Cl trend, Trend 3. We conclude that the behaviour of Na and Ca in most formation waters reflects equilibration with minerals, and concentrations of Ca in solution are sensitive to pH and PCO2 as well as to chloride concentration. For some brines however, the amount of salts in solution is sufficient to overwhelm the buffering capacity of the wallrocks. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Formation water Scale formation Brine geochemistry

1. Introduction Petroleum reservoirs show considerable variability in the composition of the formation waters residing within them. Most reservoir formation waters have calcium and sodium as the major cations, but their relative proportions are very variable. Chloride is generally the dominant anion, but sulphate, bicarbonate and more rarely acetate may also be important. While saline waters and brines are the most common formation waters, dilute bicarbonate formation waters also occur, especially in non-marine settings. Formation water variability is reflected in the changing abundances and proportions of these elements and their species (Collins, 1969; Fontes and Matray, 1993; Davisson and Criss, 1996). Gaining a fuller * Corresponding author. E-mail address: [email protected] (B.W.D. Yardley). 0264-8172/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2011.03.002

understanding of the controls on Ca concentration in formation waters is of practical importance to the petroleum industry. Elevated concentrations of Ca increase the risk of carbonate or sulphate scale formation during oil production processes that involve pressure reduction and/or incompatible water injection. Furthermore, the efficacy of the novel EOR technology LoSalÔ has been shown in coreflood experiments to be dependent on the presence of divalent cations such as Ca in the formation water (Lee et al., 2010; Lager et al., 2008). Understanding the controls on the dominant formation water cations will also be important for assessing the possibility of permanent CO2 storage in the subsurface, both because they influence the solubility of CO2 in brines and because of their possible involvement in the precipitation of secondary carbonates. The object of this study was to investigate the relationships between Ca and Na contents in natural formation waters, and to identify the factors that control them.

S. Houston et al. / Marine and Petroleum Geology 28 (2011) 1242e1251

2. Controls on Na and Ca in formation waters

K1 ¼

It has long been recognised that basinal brines include both bitterns remaining after halite precipitation, which may have high levels of Ca relative to Na, and products of halite dissolution after the evaporite-forming event, in which case they are likely to be NaCldominated (e.g. Rittenhouse, 1967; Stueber and Walter, 1991; Bein and Dutton, 1993). However it has been shown by Hanor (1987, 1994) that many oilfield formation waters have a cation chemistry that is controlled by chemical equilibration with the host rocks. Thus basinal fluids are almost invariably enriched in Ca relative to Na compared to evaporated modern seawater of similar chlorinity, as the result of mineral-fluid interactions in the subsurface. The dominant modifiers of Ca and Na concentration in formation waters are generally believed to be interactions with feldspar minerals (e.g. Davisson and Criss, 1996), and reaction with carbonates (including dolomitisation), clays and sulphates (Land and Macpherson, 1992). Evidence from changes in produced water compositions from oil fields undergoing water injection have demonstrated that fluid composition can be changed by the dissolution or precipitation of minerals over a very short time scale (Houston et al., 2007). Lowenstein and Timofeeff (2008) have recently suggested however that variations in formation waters may to some extent reflect changes in seawater chemistry through time. We explore here the extent to which the compositions of basinal fluids can be considered to represent equilibration between pore waters and minerals (and therefore predictable in principal), and the extent to which they reflect the quantities of cations available in minerals and pore fluid initially. Very high cation concentrations in brines may exhaust the exchange capacity of the host rocks, especially if they are porous, quartz-rich sandstones, in which case we refer to the brine chemistry as being mass-limited. The possibility that variations in Na:Ca ratios of formation waters may reflect changes in seawater chemistry through time is evaluated in Section 8. 2.1. Mechanisms of evolution of pore waters The composition of formation water may be modified from that of seawater by a series of processes which occur at the surface prior to burial or subsequently in the subsurface. These can be grouped into four classes: 1) external processes that determine the initial pore water composition on burial or after an episode of infiltration and flushing, for example evaporation of surface waters to produce brines and bitterns. 2) alterations to water composition through mineral-fluid exchange. 3) mixing of waters of different salinity, which can itself lead to further fluidemineral interactions (Hanor, 2001) and 4) alterations to water chemistry through gas-water or oilewater interactions. Na concentration in pore waters is controlled primarily through variation in salinity, and secondarily by ion exchange with Nabearing minerals. Values of Na/Cl>1 are rare except for dilute alkaline bicarbonate waters, and, in chloride waters, significant increases in Na can only occur during diagenesis through halite dissolution (Fontes and Matray, 1993). Except in extremely dilute fluids, dissolution of Na-silicates can only yield additional Na if other cations are removed from the fluid to maintain constant charge. Calcium concentration in pore waters is commonly limited by carbonate equilibria which are influenced by variations in PCO2 . For example, calcite dissolves as a result of increasing PCO2 from oxidation of organic matter or influx of gas:

CaCO3 þ CO2 þ H2 O4Ca2þ þ 2HCO 3

(1)

The Ca content of the fluid is thus linked to the equilibrium constant K for this reaction and PCO2 through:

1243

aCa2þ ,a2 HCO 3 f CO2

where fCO2 is the CO2 fugacity, i.e. the thermodynamically effective pressure, and the standard state for CO2 is the hypothetical ideal pure gas at 1 bar pressure. Bicarbonate ion concentrations are sensitive to pH, and this may be controlled independently, for example by interactions between clays and feldspars. Two major diagenetic processes that modify the Ca:Na ratio of formation waters are dolomitisation of calcite and albitisation of detrital plagioclase feldspars. Both these processes add Ca to the fluid, in place of either Mg or Na:

2CaCO3 þ Mg2þ ¼ CaMgðCO3 Þ2 þCa2þ

(2)

CaAl2 Si2 O8 þ 4SiO2 þ 2Naþ ¼ 2NaAlSi3 O8 þ Ca2þ

(3)

Davisson and Criss (1996) devised a plot in which the excess Ca present in formation waters relative to concentrated seawater of an equivalent chlorinity, is compared with the deficit in Na, and showed that many formation waters lie on a trend indicating that Ca increases as Na deceases. Although this has been interpreted as the result of a 2for-1, Na for Ca exchange, consistent with albitisation as the dominant mechanism by which Ca is added to formation waters, this is controversial because it follows from charge balance that waters dominated by Na, Ca and Cl must lie on such a trend. Of course, many basins preserve evidence of widespread dolomitisation in the geological record, so dolomitisation must also be an important cause of increased Ca in pore waters, especially in carbonate-rich sequences. Formation waters may originate as seawater but may also begin as more concentrated brines as a result of evaporation at the surface. Such brines sink, displacing less dense seawater-derived pore fluids, and so have a relatively high preservation potential. In the plots presented here (Figs. 2e5 and 8), we have shown seawater evaporation trends based on the maximum Ca concentration that might result, by taking Ca-concentrations for evaporated modern seawater from Fontes and Matray (1993), and adding the molal Mg-concentration from the same data set to give the maximum Caconcentrations that could be achieved by brines that have undergone complete loss of Mg by dolomitisation. For many host rocks, the relative proportions of Na and Ca will ultimately be controlled by equilibrium between Na- and Cabearing secondary silicate minerals, rather than reflecting specific diagenetic processes, for example:

Al2 Si2 O5 ðOHÞ4 þ4SiO2 þ CaCO3 þ 2Naþ ¼ 2NaAlSi3 O8 þ 2H2 O þ CO2 þ Ca2þ

(4)

For which:

K4 ¼

aCa2þ ,f CO2 a2 Naþ

Absolute concentrations of Ca and Na are further constrained by charge balance, predominantly with chloride in most formation waters. It follows from the form of this equilibrium constant that at higher levels of Cl, the ratio Ca/Na must increase. Equilibrium (4) is likely to provide an important compositional limit in many clastic sediments containing carbonate shell fragments or calcite cement, following initial reaction to generate diagenetic albite and kaolinite, but mineralogical controls on Ca-concentrations are not limited to silicateecarbonate interactions. Sulphate varies independently of the chloride concentration in formation waters (Hanor, 1994). In the case of the global data set used in this study, SO4:Cl ratios vary over five orders of magnitude. Where there is sufficient dissolved sulphate (for example in buried seawater), the increase in temperature associated

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with burial of formation waters may cause them to become supersaturated with CaSO4 (Holland and Malinin, 1979). Gypsum or anhydrite commonly precipitate in such situations, causing the concentration of Ca in the formation water to be reduced, eg: 2þ SO2 þ 2H2 O4CaSO4 ,H2 O 4 þ Ca

(5)

Diagenetic reactions may lead towards equilibrium buffers constraining formation water chemistry, but some reactions may be limited by kinetic or mass-balance constraints rather than buffers. Where Ca and Na concentrations are affected by the interaction of the fluid with both halite and silicate minerals, the relative volumes of the minerals and the fluid (related to the porosity and flow through the system) may be important. This is discussed further below. However, where formation waters are in equilibrium with both a Naphase such as albite and a Ca-phase, including calcite, then it can be seen from reaction 4 above that the result of mineral buffering will be fluids with relatively constant values of (aCa2þ/a2Naþ), depending in detail on temperature and, if the Ca-phase is calcite, PCO2. This study has involved the compilation of an extensive data set of formation water chemistries with a view to establishing if there are general trends in patterns of Ca and Na concentrations, and identifying the processes likely to lead to them. 3. Data set The data set used for this study is summarised in Figure 1, which also is the key to the symbols in subsequent figures, and in Table 1.

It comprises a range of literature data with additional results provided by BP from new or currently producing oil fields. Most of the unpublished BP data has been made available as a Supplementary Data File that accompanies this paper on the journal web site. It includes analyses from both onshore and offshore basins, including the North Sea and Gulf of Mexico regions. Some of the data have been used in earlier compilations such as Davisson and Criss (1996), but we have deliberately not used all the data sets available for the southern USA and Gulf of Mexico, as this would have resulted in plots dominated by data from a relatively small geographical area in which almost all of the formation waters are very highly saline. The quality of the data is variable due to the range of sources and variety of analytical techniques. Data obtained from the literature is likely to have been subject to quality control prior to publication, but some data sets do come with caveats. Samples from working oil fields may be obtained through a number of different processes, some of which can result in contamination. These include: drillstem tests (pressurised and uncontaminated); other down-hole tools (pressurised but may be contaminated) and well-head and separator samples (depressurised). Every effort has been made to reduce uncertainty and to use only the most reliable analyses. Each analysis was charge balanced and those that showed more than 1% discrepancy were generally rejected. This rule was relaxed for the very dilute waters from the San Juan basin and Colombia, because in these cases a very small inaccuracy in the analysis and speciation of carbonate can lead to a big charge balance discrepancy. This

Figure 1. Summary of the sources of the data presented in the subsequent figures, showing the symbols used for each data set. The full analyses are available in the supplementary data file or in the following publications: Connolly et al.,1990a,b; Collins et al., 2004; Carpenter et al.,1974; Bazin et al.,1997; Warren and Smalley, 1994; Egeberg and Aagaard,1989; Land et al., 1988; Hyeong and Capuano, 2001; Kharaka et al., 1977; Snyder et al., 2003; Jensen et al., 2006; Lowry et al., 1988; Wilson and Long, 1993a,b; Knauth, 1988; Stueber and Walter, 1991.

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Table 1 Summary of the basins from which data was used in this study, including the approximate age of the sediments and the trends (see text) on which their Ca:Na analyses fall. Basin

Age of sediments

Evaporites Present

Trends in data

Present Location

Alberta

dev-miss þjur-cret cret-olig cret-pal cret-pal jur-cret cret-eo lower sil sil-dev

1

onshore

1 1 1 1 3 1,?3 1,2

offshore offshore offshore onshore onshore offshore onshore

mio-plio sil-dev dev-eo jur-mio jur-mio jur-mio jur-mio jur-mio

minor, in ord and miss units yes no no yes no yes minor, in ord and miss units no yes In places yes yes yes yes yes

3,1 1,2 1,2 1,2,3 1,2 1 1,2 2

onshore onshore offshore offshore offshore offshore offshore offshore

carb-tri cret dev

yes no yes

1,2 3,1 1,2

onshore onshore onshore

Angola Azerbaijan 1 Azerbaijan 2 Central Mississippi Columbia Eastern Ohio Illinois Mahakam Michigan North Sea Offshore Gulf of Mexico 1 Offshore Gulf of Mexico 2 Offshore Gulf of Mexico 3 Offshore Louisiana Onshore Gulf of Mexico Texas Palo Duro San Juan Saskatchewan

Notes: sil - silurian, dev - devonian, miss - mississippian, tri - triassic, jur - jurassic, cret - cretaceous, pal - paleocene, eo - eocene, mio - miocene, plio - pliocene.

study focuses on first order trends which are likely to be robust irrespective of minor inaccuracies and scatter. Furthermore Ca and Na analyses are among the most robust, although estimates of PCO2 are more susceptible to error. Analyses are presented in a range of units in the literature; in this paper we principally use molal units (moles solute per kg of solvent) and where necessary these were calculated from analyses given in mg/l. The mass of the total dissolved load was subtracted from the mass of 1 l of fluid to obtain the weight of solvent by difference. Where specific gravity (SG) values were not available, they were estimated from the chloride concentration in mg/l using the relationship: SG ¼ 1 þ (Cl  1.133E-06). This relationship generally agrees with measured values to better than 1%. The results are presented as calculated from the cited sources. 4. Results Concentrations of both Ca and Na are expected to increase with salinity and this is confirmed for the global data set in Figures 2 and 3. Linear and log-scale plots are provided in each case. When Na is plotted as a function of Cl the data follow a straight line, Na:Cl ¼ 1, at moderate salinities (Fig. 3). However at very low and very high Cl values the relationship is less clear cut and Na concentrations may fall above and below the line respectively. At higher salinities (above 3.5 molal Cl) some data from Central Mississippi, Saskatchewan and the Gulf of Mexico have Na/Cl ratios <1, representing brines with a significant CaCl2 component. At very low-Cl concentrations, waters from the San Juan basin, Colorado, exhibit anomalously high Na/Cl ratios due to the presence of sodium carbonate species in solution. In contrast, waters with similar low chloride concentrations from Colombia mostly lie close to the Na:Cl ¼ 1 trend. Similar plots for Ca against Cl show likewise that at low chlorinities, bicarbonate-rich fluids yield anomalously high Calevels relative to the general trend of the data from higher salinities (Fig. 2b). At Cl greater than 2 molal, the data split into two trends with ratios of w 0.25 (rapid increase in Ca with Cl, seen in data from a wide range of locations summarized in Table 1) and w 0.05 (slow increase in Ca with Cl, predominantly Gulf of Mexico data, Table 1).

Figure 2. Variation in Ca concentrations of formation waters with Cl. Note two trends forming at higher salinities on the linear scale plot (a). At very low chloride levels, shown more clearly on the log plot (b), Ca varies independently of Cl due to the dominance of other ligands. The dashed line across the bottom of part (a) is the seawater evaporation trend from Fontes and Mattray (1993), without any allowance for diagenetic modifications, and emphasises the strong enrichment in Ca of almost all pore waters.

Figure 2 also includes the seawater evaporation trend discussed above; for chloride molalities below halite saturation, many data points lie above the trend, i.e. the fluids are more enriched in Ca than could be achieved through dolomitisation at a given chlorinity. In contrast, at higher chloride levels, all data points lie below the trend. This suggests that albitisation is a major process enriching formation waters in Ca for moderate salinity fluids (Davisson and Criss, 1996), enhancing Ca-levels beyond what could be achieved by dolomitisation, but extremely saline fluids which contain very high Ca-levels could arise simply through dolomitisation, since Ca þ Mg concentrations are very high in bittern brines. A major role for dolomitisation by bittern brines in the development of high Cl,

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Figure 3. Variation in Na concentrations of formation waters with Cl. Waters show a strong 1:1 positive correlation (show by the dashed diagonal) although the linear plot (a) demonstrates deviations due to relative Na-deficiencies at moderate to high salinities in many samples, while the log plot (b) illustrates that dilute fluids may have a marked excess of Na due to the presence of Na-bicarbonate. In both parts it is apparent that CaCl2-rich brines closely track the trend for highly evaporated brines after their involvement in dolomitisation.

high-Ca fluids is also indicated by the behaviour of Na; the data lies very close to the modelled trend for bittern brines after dolomitisation in Figure 3. This will be discussed further in Section 7. The molar fraction XCa (Ca/Ca þ Na) reflects the relative importance of Ca and Na in solution. XCa will increase with Cl if mineral buffers control the Na:Ca relations through equilibria such as (4) above. When plotted against chloride, the XCa data form three distinct trends (Fig. 4). The first trend is characterised by XCa increasing steadily with salinity, i.e. Ca gradually becomes a more dominant cation as salinity increases (Trend 1, high-Ca). This trend is apparent for moderate to high salinity fluids on the linear plot (Fig. 4a), but the log plot (Fig. 4b) demonstrates that it actually

Figure 4. XCa (Ca/(Ca þ Na)) variation with total chloride. a) Linear plot showing that most data define a trend of increasing XCa with Cl (Trend 1), but some high Cl fluids have very low XCa (Trend 2) while very dilute fluids show a large range of XCa values (Trend 3). b) Log plot demonstrating that Trend 1 continues to Cl levels below seawater concentration. Note the spread of data between Trends 1 and 3, reflecting meteoric infiltration into onshore basins, and the paucity of intermediate data points between Trends 2 and 3.

continues through to fluids less saline than seawater in some basins. A second group of data exhibits little or no increase in XCa with salinity, i.e. Na remains dominant up to the very highest salinities (Trend 2, low-Ca). The existence of two such distinct trends was previously reported from the Gulf Coast by Hanor and McIntosh (2007). Trend 3 (low-Cl) comprises dilute waters, often rich in bicarbonate, which can exhibit very large variations in XCa. The log plot (Fig. 4b) demonstrates that fluids with less than 0.1 m Cl invariably have XCa < 0.1 but there are systematic differences between the two fields with low-Cl waters included here, with consistently higher XCa for fluids from Colombia compared to those from the San Juan basin.

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Additionally, some onshore fields show evidence of dilution of brines by meteoric waters, and this is seen in Figure 4 as a scatter to the left of Trend 1.

5.1. Trend 1 (high-Ca) Data from most of the fields studied includes points that lie along Trend 1, which continues to Cl-levels below those of seawater (Fig. 4b). There are two ways in which this trend might arise. Mineral buffering (Hanor, 1987) fixes the ratio (aCa2þ/a2Naþ) (equilibrium 4, above) and so as Na increases with increasing Cl, Ca must increase more rapidly if this ratio is to be maintained in waters of different chlorinity. Hence this trend of increasing XCa with salinity conforms to the effects of buffering by equilibrium with Na and Ca minerals. At very high chlorinities, seawater evaporation followed by MgeCa exchange through dolomitisation can also produce a similar effect since the seawater evaporation with dolomitisation trend rises steeply on Figure 4 above c. 5molal Cl. Analyses from the Michigan Basin data sets closely track the trend for bittern brines after dolomitization on both Figures 2 and 3 also, and since they often have anomalously high K/Na ratios, this may be the explanation for the high XCa values found in this data set and some other very concentrated calcic brines. If mineral buffers are important, the ratio aCa2þ/a2Naþ should remain reasonably constant irrespective of salinity, although if calcite is the Ca-phase present, there will be some dependence on PCO2. We have treated the analytical Na and Ca values as proxies for cation activities, itself a significant oversimplification for the more saline fluids. Nevertheless, a plot of log ((mol Ca)/(mol Na)2) against Cl (Fig. 5), shows that this ratio generally lies between 2.0 and 0.5 for those waters in Trend 1 that have chlorinities either below or slightly above halite saturation at surface conditions. The increase in this ratio at high salinities may reflect the greater deviations from ideality to be expected for Ca relative to Na, but could also be a consequence of increased Ca in the fluid through dolomitisation. The fit is not perfect, but even in formations where a buffering assemblage is present, scatter may arise due to non-ideal behaviour (especially in the most concentrated waters) and through variations in pH and PCO2. It appears therefore that the relative proportions of Na and Ca in most saline oilfield formation waters lie along Trend 1 and are mainly controlled by mineral buffering.

5.2. Trend 2 (low-Ca) Figure 5. Molar Ca/Na2 ratio as a function of Cl. a) shows fluids to 6 m Cl only and demonstrates that except at very low Cl values this parameter is rather constant, consistent with mineral buffering. In b) the full data set shows deviations to higher and lower values at very high Cl. While some variation is to be expected due to non-ideal behaviour, it is unlikely that this effect would produce both higher and lower values.

5. Interpretation The three contrasting trends in NaeCa behaviour documented in Figure 4 can be attributed to different types of fluiderock interactions and different proportions of components in the original system. While some fields have data which lies entirely along one of these trends, many fields have individual points that lie along more than one of them (Table 1). In particular, a number of fields with high-salinity brines associated with evaporates include data from both the high-Ca and Low-Ca trends. This is particularly apparent for the extremely concentrated fluids from Saskatchewan, but is also observed in data from the offshore Gulf of Mexico.

Data from a number of fields includes arrays of points for which XCa increases little, if at all, with salinity, and log (mol Ca/mol Na2) decreases slightly with salinity (Fig. 5b) so it is unlikely that mineral interactions are controlling the Na:Ca relationships. Some of the highest salinity brines in the data set exhibit this type of behaviour, notably those associated with salt diapirs in the Gulf of Mexico. We interpret this trend as arising where large amounts of Na were introduced into the formation waters as halite dissolved, so that there was insufficient Ca present in minerals to be able significantly to increase the proportion of Ca in the pore fluid, even if complete albitisation took place. We conclude that the pore water compositions on Trend 2 are not controlled by mineral buffers but are limited by the relative amounts of Na and Ca initially present in the host rock and introduced through halite dissolution into the pore water. We term this control “mass-limiting” since it results in fluid compositions that reflect the mass balance of solid and fluid inputs, rather than the attainment of chemical equilibrium with buffering assemblages. It is evaluated further below.

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5.3. Trend 3 (low-Cl) Many low-salinity formation waters tend to be HCO3 dominated (Hanor, 1994) and for most of the data set there is an inverse correlation between HCO3 and Cl which may be due to a salting-out effect; i.e. dissolved salts lower the solubility of gas species in water affecting HCO3 concentration (Fig. 6a). Only at very low chloride levels is this trend absent. At low temperatures CaCO3 is more soluble than Ca-silicates (e.g. Maher et al., 2006) and low temperature, low-salinity waters are often dominated by dissolved calcium bicarbonate, with high values of XCa. Many low-salinity formation waters can be considered similar to shallow meteoric groundwaters. An example of a basin which displays trend 3 behaviour is the San Juan basin in Colorado/New Mexico, USA (Riese et al., 2005).

a

1.0E+00

HCO3 (m)

1.0E-01

1.0E-02

1.0E-03

The formation waters hosted by the Fruitland Formation coals tend to be low-salinity, bicarbonate-dominated and rich in Na, Ca and Br, though there is some areal variation. The high Br content is attributable to interaction with organic matter. The Ca and HCO3concentrations are controlled by equilibrium with carbonate minerals, while the high Na content is due to interaction of the formation water with Na-bearing clays, including bentonite. The low overall salinities are believed to arise through meteoric recharge into formations with original brackish pore waters (Kaiser et al., 1994; Snyder et al., 2003; Riese et al., 2005). Low-salinity waters from Colombia and the Mahakam Basin also lie on the low-Cl waters trend and indicate that equivalent processes are occurring in these locations despite significant differences in age, temperature and mineralogical composition. Colombian formation waters have extremely high XCa and Ca/Mg ratios and their speciated Ca and Mg values suggest that the equilibrium carbonate in the system is calcite rather than dolomite (Fig. 6b). These characteristics may arise because quartz and small amounts of feldspar, clays and carbonates are the only minerals available to interact with the fluid (Cazier et al., 1995). In conclusion, it is likely that Trend 3, like Trend 1, represents mineral buffering, but in an environment where low-Cl levels allow bicarbonate equilibria to dominate the fluid chemistry. Our interpretation of the relationship between trends 1 and 2 differs from that of Hanor and McIntosh (2007) who suggested that similar trends in Gulf Coast sediments were linked to the evolution of NaCl brines through progressive equilibration. The clear separation of the trends even when a global data set is considered make this unlikely in our opinion. Furthermore we show below that high XCa Trend 1 brines have distinctive Cl/Br ratios. 6. Evaluation of mass-limited and mineral buffered Trends

1.0E-04

1.0E-05 1.0E-05

b

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

Cl (m)

1.0E+01

0.004

0.0035

1/T K

0.003

0.0025

0.002

0.0015

0.001 -1.0

Magnesite and solution

-0.5

0.0

0.5

Dolomite and solution

1.0

Calcite and solution

1.5

2.0

2.5

log Ca/Mg (Mol) Figure 6. a) Bicarbonate shows a distinctive inverse correlation with Cl at moderate to high salinities, but the lowest chloride fluids have high but rather uniform bicarbonate concentrations. b) Relation between Ca/Mg and temperature, showing experimental data for low temperature carbonate equilibria for comparison (Holland and Malinin, 1979).

Saline formation waters have been shown to lie along one of two distinct trends, one is interpreted as mainly arising through mineral-fluid interactions, leading to a mineral buffered fluid composition, the other is due to dissolution of halite proceeding to such an extent that the buffering capacity of the sediment is overwhelmed. To demonstrate these alternative controls on brine chemistry, a simple model has been constructed to illustrate the relationships between the high-Ca Trend 1 and the low-Ca Trend 2. The starting point is a quartzofeldspathic host rock with 10% pore fluid by weight, prior to diagenetic alteration. Although plagioclase is now rare in many oilfield sandstones it is sometimes found, for example in sediments of the US Gulf Coast (Land and Macpherson, 1992), and may have been more common prior to albitisation during diagenesis; thus plagioclase (composition An20) is taken as the mineral source of Ca in the model. Figure 7 explores the relationship between the amount of plagioclase in the original sediment, the chlorinity of the fluid and the final XCa value of the fluid for a constant porosity and plagioclase composition. The fluid composition has been calculated from the quantities of Ca and Na in the system, assuming that all the plagioclase has been entirely altered to albite, ie there is no mineral buffering because the Caphase was exhausted. For any given amount of plagioclase in the rock, the relative importance of Ca in the final fluid decreases as the salinity of the pore water increases (Fig. 7a), while the proportion of Ca in the pore water increases with the amount of plagioclase originally in the rock. Figure 7b contrasts the variations in fluid composition (XCa) to be expected with increasing salinity according to the mass limiting model of Figure 7a with the trend to be expected from mineral buffering. The mineral buffering trend is simply calculated on the basis of an arbitrary constant value of (mol Ca/mol Na2), whereas the mass-limited trend is for a rock with 10% porosity and 30% An20 plagioclase at the outset. Note that the

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Figure 8. Plot of XCa versus molar Cl/Br, note that data must be compared with Figure 4 for information about fluid salinity. Bittern brines with Cl/Br < seawater show a wide range of XCa values, lying on Trend 1(Fig. 4) whereas Ca-poor brines of Trend 2 commonly show marked Cl-enrichment due to halite dissolution.

Figure 7. a) Model illustrating the importance of Ca-bearing minerals in systems associated with salt. The model is based on a siliceous rock with 10% porosity by weight and a varying proportion of Ca-plagioclase which is altered to albite releasing Ca into a pore fluid of variable salinity. The relative importance of Ca in the final fluid decreases as amount of salt increases. Overall, the proportion of Ca in the pore water increases with the amount of feldspar. b) Two models describing the behaviour of Ca in formation waters. The red line represents a mass-limited trend where all the Ca in the original reservoir minerals is replaced by Na so that the relative concentration of Ca in the final fluid decreases with increasing chlorinity. The blue line represents a generic buffering trend for which there is a sufficient supply of mineral Ca to maintain a constant value of (mol Ca/mol Na2). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

mass-limited model yields very Ca-rich fluids if the salinity is extremely low, because all Na is fixed in albite while Ca is released to the fluid. With higher salinities, the two trends cross and follow paths that are very similar to the shapes of Trends 1 and 2 in Fig. 4. 7. Origin of high-Ca hyper-saline brines A striking feature of the very high salinity brines included in this study is that they divide very clearly into a Ca-rich and a Ca-poor group. When XCa is plotted against Cl/Br (Fig. 8) it is clear that the

low XCa sodium chloride brines are also anomalously low in Br, indicating an origin by dissolution of halite, while the high XCa fluids are all Br-enriched bitterns. Some of the low-Br brines are extremely concentrated, but others are not, consistent with the variety of chlorinities encountered today in the vicinity of allochthonous salt sheets of the Gulf Coast (Hanor and Mercer, 2010). Ca-rich brines and hyper-saline brines lie very close to the trend on Figure 8 for evaporated seawater that has participated in dolomitisation. It seems likely from the close correspondence between high Ca-levels and low-Cl/Br ratios that bittern brines are readily involved in dolomitisation. We infer that these Br-rich brines originated with evaporites in the basin and subsequently dolomitized limestones in the subsurface, so that the present Ca content may approach the original level of Ca þ Mg (in molar units). The close correspondence between the analysed brines and the trend for Ca þ Mg of evaporated modern seawater on Figure 8 not only eliminates the case for Ca-rich brines arising from evaporation of CaCl2-enriched seawater during specific geological periods (Lowenstein et al., 2003; Lowenstein and Timofeeff, 2008), it indicates that all Phanerozoic seawaters evolved along the same trend of Ca þ Mg versus Cl/Br as modern seawater (Fontes and Matray, 1993), implying that at least the Ca þ Mg content of Phanerozoic seawater has remained more or less constant. An additional issue with some hyper-saline high-Ca brines is how they acquired their exceptionally high chlorinities. While very high chlorinities of brines derived by dissolution of halite in the subsurface can be accounted for if that dissolution took place at elevated temperatures, it is difficult to see how subsequent salt dissolution could lead to the enhanced chlorinities of the calcic brines, which can exceed chloride saturation at surface temperatures, without having a major impact on XCa. An alternative explanation is that these brines dissolved bittern salts in the subsurface, or simply lost water, rather than gaining salts. This might be achieved by interaction with a gas phase at elevated temperatures after deep burial or alternatively, water may have been lost to clays. Both these mechanisms have the effect of dehydrating the brine.

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Figure 9. Schematic representation of the possible patterns of evolution of sediment pore waters into formation waters of Trends 1, 2 or 3 as identified here. Salinity increases to the right while the vertical axis represents depth and passage of time. The boxes denote interactions with different types of solid material while the arrowed paths show how fluids of different original salinities can evolve into the distinct Trends through interactions with halite, silicate rocks or limestone.

8. Discussion and conclusions Our analysis of relationships between Ca and Na in natural oilfield formation waters has demonstrated 3 main global trends that reflect the different controls on pore fluid chemistry. Trend 1, a highCa brine trend, is marked by increasing XCa with Cl. We believe there may be two mechanisms that can lead to brine compositions along this trend. At moderate salinities, when both Ca- and Nabearing minerals are present, then the ratio (aCa2þ/a2Naþ), and its proxy (mol Ca)/(mol Na)2, is approximately constant due to mineral buffering. As a result, XCa increases with salinity and this trend is exhibited by the majority of oilfield brines. A few basins contain extremely saline bittern brines which lie on the extension of the mineral buffering trend, but are so rich in dissolved Ca that it is likely that their Ca:Na relationships are mass-limited by the amounts of cations available initially in the sediment and pore fluid. We infer from the correlation of XCa with Br/Cl that these brines owe their high relative Ca-levels to earlier involvement in dolomitisation. Some saline formation waters lie along Trend 2 (lowCa), and are low-Ca brines with rather low Ca-contents and low XCa values even at very high salinities. These NaCl-dominated waters are associated with halite deposits and their pore water chemistry is believed to arise because the quantity of Na introduced as dissolved salt is sufficiently large that it overwhelms the small amounts of Ca available from silicate breakdown so that the formation water is effectively unbuffered or mass-limited. Finally, at low salinities, formation waters are often dominated by bicarbonate and although water chemistry may be mineral buffered, the importance of bicarbonate species gives rise to very high XCa values. These waters lie along a distinct Trend 3, low-Cl waters. Although several fields studied yielded a range of brines, some lying on trend 1, others on trend 2, there are very few analyses that lie between these trends at high salinities, although there is no reason in principle that such fluid should not occur. In contrast, many onshore fields have been reported to yield evidence of mixing between formation brines and low-salinity meteoric waters, and

this is reflected in the spread of the data, for example on Figure 4b. Table 1 is a summary of the characteristics of the formation waters of all the basins considered in this study, indicating which compositional trends are mainly represented in each. This study has confirmed that the major element cation chemistry of a very wide range of oilfield formation waters is dictated by mineral-fluid interactions, and similar trends are displayed by waters from quite distinct basins, hosted by rocks of very different ages. The three main types of behaviour that we have documented depend first on the relative importance of bicarbonate and chloride anions, which dictates the nature of the equilibrium partitioning of Na and Ca between sediment and fluid, and second on the relative availability of Na and Ca in the sediment and its pore waters during diagenesis, which determines whether sufficient mineralogical diversity remains in the sediment during diagenesis to provide a mineral buffer. The evolution of original sediment pore water into formation waters belonging to the trends we have identified is represented schematically in Figure 9. With the much more extensive database interrogated here, we concur with the findings of Hanor and McIntosh (2006) and can find no evidence to support the proposal by Lowenstein et al. (2003) and Lowenstein and Timofeeff (2008) that aspects of formation water chemistry reflect variations in seawater through Phanerozoic time, and specifically that differences in the relative proportions of Na and Ca reflect the age of formation waters. Indeed, for some of the data sets considered here, it is clear that the formation waters are not the same age as the sediments that host them, making them impossible to date. In the Alberta basin, Connolly et al. (1990b) noted that formation waters in lower Cretaceous sediments shared similar isotopic and chemical characteristics to those in the underlying Palaeozoic sequences, but were quite distinct from those in the overlying upper Cretaceous sequence. Acknowledgements This research was carried out while the first author held a NERC Industrial CASE studentship, supported by BP, at the University of Leeds. We are grateful to BP for access to unpublished data and for permission to publish the work. Bruce Yardley is indebted to Ben Rostron for providing analyses of Saskatchewan brines. The manuscript was significantly improved by comments from Jeffrey Hanor and Volker Lüders. Appendix. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.marpetgeo.2011.03.002. References Bazin, B., Brosse, E., Sommer, F., 1997. Chemistry in oil-field brines in relation to diagenesis of reservoirs 2. Reconstruction of palaeo-water composition for modelling illite diagenesis in the Greater Alwyn area (North Sea). Marine and Petroleum Geology 14, 496e511. Bein, A., Dutton, A.R., 1993. Origin, distribution and movement of brine in the Permian Basin (U.S.A.): a model for displacement of connate brine. Geological Society of America Bulletin 105, 695e707. Carpenter, A.B., Trout, M.L., Pickett, E.E., 1974. Preliminary Report on the origin and chemical evolution of lead- and zinc-rich oil field brines in Central Mississippi. Bulletin of the Society of Economic Geologists 69, 1191e1206. Cazier, E.C., Hayward, A.B., Espinosa, G., Velandia, J., Mugniot, J.-F., Leel Jr., W.G., 1995. Petroleum geology of the cusiana field, Llanos asin Foothills, Colombia. Bulletin of the American Association of Petroleum Geologists 79, 1444e1463. Collins, A.G., 1969. Chemistry of some Andarko brines containing high concentrations of iodide. Chemical Geology 4, 169e187. Collins, I.R., Graham, G.M., Stalker, R., 2004. Optimising the levels of sulphate reduction required to mitigate the risks associated with barium sulphate scale

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